This invention relates to a variable displacement axial piston pump having a thrust plate mounted on a movable rocker cam which is supported on a rocker cradle. More particularly, this invention relates to a means for supporting the rocker cam on the rocker cradle and for retaining the cam against the cradle.
In some previous axial piston pumps manufactured by the assignee of the instant invention, the rocker cam had a convex, arcuate surface opposite the thrust plate supporting surface which was received in a complementary concave, arcuate surface formed in the rocker cradle. A plain, Teflon-coated, bronze bearing was inserted between the arcuate cam and cradle surfaces to reduce friction therebetween when the cam was pivoted in the cradle. In this machine, pressure fluid was supplied to the bearing to counterbalance the piston forces acting to force the rocker cam against the bearing to reduce the force required to pivot the rocker cam in the cradle. A problem with feeding pressure fluid to the back of the rocker cam is that there is continuous fluid leakage. This leakage results in reduced pump efficiency and a resultant power loss. An additional problem with the plain bearing supported rocker is that the coefficient of friction between the rocker cam and bearing changes with wear. This changing friction level adversely affects a rocker cam drive which has available to it only a limited amount of control pressure.
In the aforementioned pump, the rocker cam was held against the rocker cradle by a pair of bolts, one on each side of the cam, while projected through the pump housing into an arcuate groove in each side of the cam. A disadvantage with this form of rocker cam holddown is that the bolts will bend if an unusually large force tending to lift the rocker cam from the cradle is encountered.
In another type of axial piston pump utilizing a rocker cam and rocker cradle, a ladder-type or caged roller bearing is positioned between the rocker cam and cradle. A disadvantage in using this bearing is that the diameter of the rollers must be relatively large, since the cage reduces the number of rollers which can be inserted between the rocker cam and cradle.
It is desirable to provide a variable displacement axial piston pump having a rocker cam and rocker cradle in which a bearing is interposed between the cam and cradle to reduce friction therebetween and which does not require the use of a hydrostatic balancing force.
It is further desirable to provide a variable displacement axial piston pump having a rocker cam and rocker cradle which utilizes a maximum number of bearing elements between the cam and cradle to provide the maximum load capacity for a given space and permit the use of relatively small bearing elements.
It is also desirable to provide an axial piston pump having a rocker cam and rocker cradle in which a bearing element supports the cam in the cradle and the same element resists forces tending to lift the rocker cam from the rocker cradle.